Do you have a ruler? My Big Mig 049 is 3mm. It's pretty easy to tell -- it'll be metric, probably 3mm like my 049.
Standard metric sizes are M3x0.5, M3.5x0.6, and M4x0.7. Unless those Ruskies are up to tricks, it'll be one of those. Diameter is obvious if you have calipers and can be squinted out if you have a ruler (3mm is just a hair less than 1/8). You can figure the thread pitch by counting off ten or twenty threads and measuring.